1. Field of the Invention
This invention relates to an improved process for the selective vapor phase oxidation of n-butane to maleic anhydride over a catalyst comprising mixed oxides of vanadium and phosphorus, (V/P/O). More specifically, the invention relates to a process involving in situ calcination and activation of the V/P/O catalyst precursor.
2. Description of the Related Art
The vapor phase oxidation of n-butane to maleic anhydride over a V/P/O catalyst is a well established commercial process. A review of the prior art describing this process is given in U.S. Pat. No. 4,668,802. In improved processes described in U.S. Pat. No. 4,668,802 and U.S. Pat. No. 5,021,588 the reaction is conducted in a recirculating solids reactor having two fluidized reaction zones. The conversion of n-butane to maleic anhydride takes place primarily in one reaction zone and the catalyst, essentially stripped of gaseous species, is transported to the other reaction zone where it is re-oxidized; i.e., regenerated prior to being returned to the reaction zone.
The mixed oxide V/P/O compositions used as catalysts in these processes are known to the art. A review of the art describing V/P/O compositions and their preparation is given in U.S. Pat No. 4,371,702. The catalyst precursor composition is usually made by a process wherein a conventional vanadium compound in which the vanadium is in the +5 oxidation state, such as in V.sub.2 O.sub.5 or NH.sub.4 VO.sub.3, is partially reduced to the +4 oxidation state by reaction in either an aqueous or organic liquid medium. The catalyst precursor is then formed by the addition of any appropriate phosphorus compound, for example H.sub.3 PO.sub.4, refluxing to bring about reaction and recovering the catalyst precursor, usually as a hydrated vanadium phosphate, by filtration and drying or spray drying. The prior art describes the desirable range of atomic ratios of phosphorus to vanadium and also the incorporation of catalyst promoters. It is desirable that the V/P/O catalyst has good attrition resistance particularly when it is used in a fluidized bed or recirculating bed reactor. The prior art relevant to attrition resistant catalyst and its preparation is reviewed in U.S. Pat. No. 4,677,084. The production of V/P/O catalysts useful for the preparation of maleic anhydride from n-butane requires controlled calcination and activation of the catalyst precursor. This is accomplished by heating the precursor under appropriate temperature, time and atmosphere conditions to accomplish dehydration, while maintaining the average vanadium oxidation state (Vox) between specified limits. For example in U.S. Pat. No. 3,915,892 each mole of hydrated vanadium phosphate, expressed as (VO).sub.2 H.sub.4 P.sub.2 O.sub.9, evolves one mole of water at 370.degree. to 394.degree. C., while maintaining the Vox in the range of 4.1 to 4.5. The balance of the water, an additional mole of water per mole of precursor, is removed by heating at 395.degree. to 600.degree. C. in a carrier gas consisting of air or an inert gas together with controlled amounts of oxygen and hydrocarbon to provide an effluent stream containing at least one volume percent of oxygen or one volume percent each of oxygen and hydrocarbon, again while maintaining the Vox in the range of 4.1 to 4.5.